I do call it lurking.

From my dictionary re: lurking read communications on an electronic network without making one's presence known.

A good conversation ensued on twitter. 'spectator' was used well by Jen Jones injenuity

I'm not sure I like spectator either because you don't necessarily get involved as a spectator - I don't like lurking either!

a fishing metaphor might be trolling but it too has less than positive connotations! Definitely need a better verb for what we do!

Plurker stalker

lurking...